In a disc drive, data are stored on one or more discs coated with a magnetizable medium. Data are written to the discs by an array of transducers, typically referred to as read/write heads, mounted to a radial actuator for movement of the heads relative to the discs. The data are stored on a plurality of concentric circular tracks on the discs until such time that the data are read from the discs by the read/write heads. Each of the concentric tracks is generally divided into a plurality of separately addressable data sectors. The transducers are used to transfer data between a desired track and an external environment, which includes, among many components, a host computer.
During a write operation, data are written onto the disc track. Once data are written to the disc, each sector holds a block of data, which is the absolute smallest quantity that can be written to the disc during a single write operation. Adjacent blocks, commonly referred to as data segments are typically written to the disc during a single write operation referred to as a command. Critical to both of these operations—reading and writing—is the accurate locating of a transducer over the center of the desired track. During a read operation a transducer senses the data previously written on the track and transfers the data to the external environment.
In general, there are two types of data stored on disc drives, normal computer data and audio/visual data. Audio/visual data relates to computer readable information stored on disc drives wherein the data corresponds to information that produces audio signals and/or visual signals. These audio and visual signals are used by a computer host to translate them into audio and video presentations through either a speaker or a monitor. Normal computer data is considered to be “reliability critical” wherein correct data storage and retrieval is much more important than any loss in time associated with achieving such reliability. In contrast, audio/visual data is considered to be time, or performance, critical. If some of the audio/visual data is corrupt and unreadable, typically the presentation to the user is not seriously affected. Often such a loss in data may present only a flicker in the audio or video signal to the user or otherwise be undetectable. However, if the information is presented with many pauses or skips due to losses in time associated with trying to insure data reliability, the resulting presentation to the user is unsatisfactory.
The transfer of files between a disc and a host computer is controlled in a multi-level setting characterized by a bi-level transfer scheme. At a macroscopic level, track sectors are selected that contain the data sectors of which the file is divided. More specifically, and in a microscopic sense, cells along a track are magnetized to correspond to the bit structure of the file for the purposes of subsequent reading. A disc drive typically includes a buffer to implement this bi-level transfer scheme. The purpose of the buffer is to accept the sectors of data during its transfer between the host computer and the disc and then transfer the data to the proper component—either the host computer or the disc.
When a disc drive is manufactured, it will typically contain a number of defects due to process imperfections and impurities or irregularities on the surface of the disc. These defects typically result in “hard” errors during read operations thereby rendering the defective areas of the disc permanently unusable. Hard errors are errors encountered during read operations that are permanent in nature in that the defective sector is permanently unable to participate in either form of data transfer—reading or writing. As such, methods are employed to “map out” the defects on the discs. Typically, a newly manufactured disc drive will be tested to determine which sectors of the discs are defective. The sector numbers of these defective sectors are then compiled into a primary defect list that is then stored, typically on a reserved area of the disc.
In addition to the primary defects that are addressed during the manufacture of the disc drive, there are also “grown defects” which occur during the operational life of the disc drive. “Grown defects,” also result in “hard errors” during read operations and are therefore permanent in nature. As with primary defects, a list of “grown” defects, sometimes called a secondary or grown list, is also maintained and stored in a reserved space on the disc. When the disc drive is powered on, typically the primary and secondary lists are read from the disk and stored in some form of random access disk drive memory. The system controller then uses the information from the primary and secondary lists to manage the defects and avoid writing data to defective sectors.
In addition to various forms of “hard errors” described above, disc drives may contain “soft errors” that further hinder disc drive performance. Soft errors are non-permanent in nature and may only occur during a single revolution of the disc. For instance, when accessing a file pursuant to a read command, a “soft error” may occur thereby rendering a particular sector of the file is inaccessible. However, that sector may be accessible to subsequent read commands or upon subsequent revolutions initiated during a read error recovery procedure of the present read command. “Soft errors” are sometimes caused by electrical phenomena surrounding the disc. “Soft errors” may also be caused by disc vibration or shock.
One method commonly used for managing defective sectors in a disc drive involves mapping each defective sector on the disc to a corresponding good substitute sector located elsewhere on the disc. However, this “mapping” technique is only useful if the error is a “hard error.” Indeed, if the error located on a sector is a “soft error,” mapping to a substitute sector would permanently render that sector useless even though the sector may be non-defective and accessible to subsequent read commands. Conventional methods employing read error recovery procedures immediately suspend the read operation when a “soft error” is encountered. Following a complete revolution of the disc, the sector having the “soft error” is positioned under the read/write head and the disc drive retries the read operation at the previously defective sector. Again, if the soft error is still present, conventional methods repeat the suspension and retry process until the read operation is successful. Once recovery is successful, the read command is executed until either another “soft error” is encountered or the end of the file being read is reached.
Conventional disc drives are typically not associated with more than one “soft error” per track. However, the potential for multiple errors on a track increases as the demand for disc drive use in an audio/visual environment increases. With respect to audio/visual data, a budgeted amount of time is reserved for read error recovery procedures due to the time-critical nature of such data. Because conventional “soft” read error recovery procedures are performed serially, ie, recoveries of defective sectors are performed one sector at a time, if multiple errors are encountered on a track, multiple revolutions of the disc are administered to recover the data from the defective sector. Hence, the budget of time reserved for read error recovery procedures is quickly depleted under such circumstances. Accordingly, because of the budget, data contained on a significant amount of sectors, both defective and non-defective, may not be transferred to the host as requested by the read command.